Jake Gyllenhaal looks positively relieved after rattling off a string of sexy puns related to the function of the male organ. “It’s probably best to get that over with,” he smiles wryly. For while the actor aggressively pursued the role of a Viagra salesman in Edward Zwick’s Love and Other Drugs – a part, he says, which comes closest to his own personality – it’s only now that the media-shy Gyllenhaal is realising the full implication of taking on such a character, and the inevitable questions that it will provoke about his own love life and sexual performance. Still, Gyllenhaal takes it like a man.

Based on the 2005 non-fiction book Hard Sell: The Evolution Of A Viagra Salesman, in which author Jamie Reidy chronicles his experiences as a young Pfizer salesman in the late nineties, Gyllenhaal spent many hours with Reidy, mastering the art of the slick pitch. “Playing a salesman felt like an old shoe – it really did,” Gyllenhaal says. “How he sold to doctors, how he would charm doctors. One of the first scenes that we shot was me pitching to a roomful of businessmen. I just found myself thinking, ‘God! I’m really good at being shallow!'”

With his striking big blue eyes, and chiselled leading man features, Gyllenhaal could have just been another pretty boy but, determined to escape typecasting, he took on a string of different roles with The Good Girl, Jarhead, Zodiac and Proof, even trying his hand at blockbuster action adventure with Prince Of Persia. Perhaps the most defining moment of Gyllenhaal’s career, however, has been his role as Jack Twist in Brokeback Mountain‘s tale of forbidden love between two cowboys.

A monogamous kind of guy, Gyllenhaal relished playing Love & Other Drugs‘ charming womaniser. Without spoiling the plot, his character is faced with tough questions about whether one would knowingly enter into a relationship with someone who was very sick. It’s a tough choice for anyone, and certainly for the actor. “The irony is that in any relationship, you have to face those questions, and it’s interesting having to face them at the beginning,” Gyllenhaal offers. “It brought up lots of questions for all of us. Anne Hathaway’s character being sick is such a huge part of the movie, but we all fall apart. It’s just the nature of being a human being. Your body deteriorates, and that’s the fact.”

Working on the film has understandably given Gyllenhaal unique insights into the pharmaceutical industry, and whether or not we’re an overmedicated society which thinks that everything can be cured with a pill. “Maybe we’re all too trusting,” the actor suggests. “We’re not listening to our hearts. I don’t want to make a blanket statement about that, but in the movie, my character says to Anne’s character, ‘You’re my little blue pill’, so perhaps whatever we need in a pill we can get in intimacy instead maybe.”

Gyllenhaal certainly agrees that love may be the most powerful drug of all. “Yeah, I’d agree, if I’m a human being,” he smiles. “I sure hope that I am. If you have it and it’s real, then it doesn’t matter how long you have it for. You can’t really fake that.”